chapter 19 Large Animal Radiography

INTRODUCTION

Working with large animal patients requires much patience and time. Any procedure performed must be well thought out before it is started. The radiographer must also expect the unexpected. Successful large animal radiography is the result of forming a plan before the examination, teamwork during the examination, and patience throughout.

Although the differences between large and small animals are great, the principles of radiography are essentially the same. All directional terms and positions that apply to a dog and a cat apply to a horse and a cow. The two major differences are size and posture. In large animal radiography, unless the animal is young or small enough to be placed on an x-ray table, the patient is in a standing position. The size and posture of the patient necessitate special consideration in the areas of patient restraint, equipment, patient preparation, radiation safety, and positioning devices.

SPECIAL CONSIDERATIONS

Patient Restraint

A large animal is often startled by unfamiliar objects, especially those brought close to its body. A good prelude to a radiographic examination of a large animal is an official introduction of the patient to the x-ray machine. Allowing the horse or cow to gently sniff the machine and cassette may eliminate fear of the unknown. Always avoid sudden movements or loud noises, which may startle the animal. Continually reassure the patient in a calm voice.

In a standing position, the large animal patient is relatively unrestrained. Because of this, there is a greater risk of injury to personnel and to the x-ray machine. The x-ray tube is particularly vulnerable because it must be positioned close to the animal’s leg and is liable to be kicked.

Several methods can be used to restrain a large animal for a radiographic examination including a twitch, stocks, and sedation. Sedation is a common method of restraint. The patient is given a small amount of chemical sedative to allow the radiographer freedom to move the x-ray machine without startling the animal, which would result in movement. If sedation is not possible or if the patient is restless, movement can be restricted if an attendant holds up one of the animal’s legs. When attempting to radiograph a limb, the opposite limb is raised. Rarely, it may be necessary to place the patient under general anesthesia. Many attendants are required to manipulate the patient and to position the equipment when the large animal is anesthetized. The veterinarian must assess the situation and determine the type of restraint necessary.

Equipment

The radiographic machinery required for large animals must have adequate power and easy maneuverability. The x-ray tube must be able to move horizontally around the standing patient and vertically to expose an area as low as the level of the floor. The x-ray machines used for radiography of large animals fall into three categories: (1) small portable units, (2) mobile units, and (3) mounted units.

The portable unit is commonly used by equine and bovine veterinary practitioners who make “house calls.” The portable unit is small and light enough that it can easily be moved from one location to another (see Fig. 2-4). The average power capacity of a portable unit is limited to a maximum milliamperage (mA) setting of 20 and a maximum kilovoltage (kVp) of 90. Due to the low mA capability, exposure times of 0.1 second or longer usually are necessary. However, long exposure times increase the likelihood of motion during exposure. Because line voltage varies from barn to barn, exposures are not always consistent with portable units. The collimation on a portable unit also varies, and the collimator may not always have a light to visualize the field of exposure. Therefore it is often easy to expose an area larger than necessary. This can pose a special problem with radiation safety (i.e., the exposure of personnel to excessive radiation).

Mobile units have the advantage of more power. The capacity of an average mobile unit ranges from 100 to 300 mA and up to 120kVp. The higher mA capacity allows for shorter exposure times. The main disadvantage of this unit is its weight and consequent lack of maneuverability. The mobile unit has large wheels to allow ambulation but tends to be cumbersome and difficult to move on uneven floor surfaces (see Fig. 4-2).

Large, permanently mounted x-ray units are commonly used by veterinary specialty and referral practices. The power capacity may exceed 1000 mA. For large animal radiography, these units are commonly mounted on the ceiling with a series of overhead rails, which allow the x-ray tube to be moved vertically and horizontally around the patient (see Fig. 2-19, B). Unfortunately, ceiling units that have overhead rails can be noisy and distracting to a fearful patient. In addition, the size of the x-ray tube housing may limit its use for studies of the feet. Even if the tube is on the floor, the focal spot may be 6 to 8 inches off the floor, resulting in obliquity of the views.

Patient Preparation

Careful preparation of the patient is necessary for an artifact-free radiograph. For all examinations, the hair coat should be brushed or washed to remove obvious dirt, bedding, and other surface artifacts. The areas of interest also should be wiped dry with a towel to remove any water or other remaining liquid contaminants. For radiography of the equine foot, a number of steps are necessary to prevent extraneous radiographic shadows over the areas under examination. The first step is to remove the shoe of the patient and trim back any overgrown portions of the hoof. Next, the sole and clefts should be picked and scrubbed clean. The final step is to pack the sole of the foot with a radiolucent material such as methylcellulose, softened soap, or Play-Doh. Packing the sole prevents the appearance of an air artifact superimposed over the areas of interest.

Radiation Safety

All rules of radiation safety discussed in Part 1 of this text also apply to large animal radiography. A few extra rules of safety must be considered, however, because of the size and posture of the patient and the considerably high exposure factors needed.

The attendants holding the patient and holding the cassette next to the anatomy must be wearing appropriate lead attire. Because the attendants’ attention is focused on the patient rather than the x-ray beam, it is the responsibility of the radiographer to ensure that all personnel are a safe distance from the primary beam. Cassette-holding devices help reduce exposure to the attendants. The device usually consists of a clamp that is attached to the cassette and held at length by a handle (Fig. 19-1).

Figure 19-1 A cassette holder used for equine radiography.

Positioning Devices

At times, it may be necessary to raise the animal’s foot because the x-ray tube cannot be dropped to the level of the floor. A positioning block can be used to raise the foot into position and to serve as a cassette holder (Fig. 19-2). The block is usually constructed of wood built to suit the particular x-ray unit. A slot can be cut into the wood to serve as a cassette holder. The foot of the patient can be placed directly onto the block to raise it into position next to the cassette, or the cassette can be placed beside the block.

Figure 19-2 For equine pedal radiography, it may be necessary to raise the patient’s foot off the ground to radiograph that area. A wood block is commonly used for this purpose. The wood block pictured here has a slot designed to hold a cassette next to the limb of interest.

Another device that is often necessary is a cassette tunnel. A tunnel can be constructed of a radiolucent wood or hard plastic, but it must be durable enough to withstand the weight of the patient. For a dorsopalmar/dorsoplantar oblique view of the coffin or navicular bone, the patient must be standing on top of the cassette. A cassette cannot withstand such weight without sustaining damage. A tunnel device can make the examination possible without damaging the equipment (Fig. 19-3).

Figure 19-3 A cassette tunnel.

DISTAL PHALANX (PEDAL BONE)

Lateral View

The patient’s foot is placed on a wood block to elevate it to a level at which the central x-ray beam can be directed horizontally toward the pedal bone. The placement of the foot must be as close to the edge of the block as possible so that the cassette is as close to the medial aspect of the foot as possible (Figs. 19-4 and 19-5). The object–film distance must be minimal. To prevent motion, it may be helpful to have an attendant hold the patient’s leg of interest over the carpus or elevate the opposite limb so that the limb being examined is completely weight bearing. The cassette is placed on the medial side of the foot, either directly on the floor or in the cassette groove in the wood block. The field of view should include the entire hoof. (NOTE: This same position is used to examine the lateral navicular bone. In that case, the beam center is directed at the palmar aspect of the coronary band.)

Figure 19-4 Correct positioning for the lateral view of the distal phalanx.

Figure 19-5 Radiograph of the lateral view of the distal phalanx.

BEAM CENTER: Over hoof wall just below coronary band

Dorsopalmar/Dorsoplantar View

The patient’s foot is placed on a wood block so that it is elevated to the level of the horizontal central x-ray beam. The heel of the foot should be placed on the edge of the block or cassette groove. The cassette is placed directly behind the foot on the floor or in the cassette groove and held perpendicular to the floor (Figs. 19-6 and 19-7). It may be helpful to raise the opposite limb so that full weight is placed on the limb of interest. This decreases the possibility of motion. The field of view should include the entire hoof.

Figure 19-6 Correct positioning for the dorsopalmar/dorsoplantar view of the distal phalanx.

Figure 19-7 Radiograph of the dorsopalmar view of the distal phalanx.

BEAM CENTER: Over middle of pedal bone just below coronary band

Dorsopalmar/Dorsoplantar Oblique View

The cassette is placed in a tunnel cassette holder, and the foot of the patient is positioned on top of the tunnel. The foot should be in the center of the cassette so that the entire hoof and pedal bone are included in the field of view (Figs. 19-8 and 19-9). It may be necessary to raise the opposite limb so that the limb of interest is weight bearing. The x-ray tube is angled 45 degrees to the ground and directed at the hoof wall. (NOTE: This same view can be used to visualize the navicular bone. Because of superimposition of the navicular bone over the second phalanx, higher exposure factors are necessary to visualize this area, and an angle of 65 degrees off horizontal should be used.)

Figure 19-8 Correct positioning for the dorsopalmar/dorsoplantar oblique view of the distal phalanx.

Figure 19-9 Radiograph of the dorsopalmar oblique view of the distal phalanx.

BEAM CENTER: Over middle point of hoof wall just below coronary band

NAVICULAR BONE

Dorsopalmar/Dorsoplantar Oblique View

The patient’s foot can be placed (1) on a cassette within a cassette tunnel, as shown for the dorsopalmar/dorsoplantar oblique view of the distal phalanx, or (2) on a block with specially designed grooves that hold the hoof at an angle (Figs. 19-10 through 19-12). With the patient standing on the cassette, the x-ray beam is angled 65 degrees toward the middle of the second phalanx. When the block is used, the toe of the hoof is placed in a vertical groove so that the dorsal wall of the hoof is positioned vertically. The cassette is placed behind the heels in a cassette groove. The opposite leg must bear the majority of the patient’s weight. The x-ray beam is directed parallel to the ground, and the field of view should include the second and third phalanges. With the foot on the block in this vertical position, a 45- to 65-degree angle view of the navicular bone is projected onto the x-ray film.

Figure 19-10 Correct positioning for the dorsopalmar oblique view of the navicular bone with the patient standing on a cassette tunnel.

Figure 19-11 Correct positioning for the dorsopalmar oblique view of the navicular bone with the dorsal wall of the hoof held in a vertical position with the use of a wood block.

Figure 19-12 Radiograph of the dorsopalmar oblique view of the navicular bone.

BEAM CENTER: Over center of second phalanx just above coronary band

Flexor View

The foot of the patient is placed on top of a cassette within a cassette tunnel (Figs. 19-13 and 19-14). If possible, the patient should be stepping back slightly so that the fetlock is in an extended position. The first phalanx is almost perpendicular to the ground in this position, allowing better visualization of the navicular bone. The x-ray tube is positioned directly behind the foot and angled approximately 65 degrees to the floor. Great care must be taken with the x-ray tube in this position immediately behind the limb. It may be necessary to reduce the source–image distance (SID) when placing the x-ray tube under the belly of a horse for views of the front navicular bone.

Figure 19-13 Correct positioning for the flexor view of the navicular bone.

Figure 19-14 Radiograph of the flexor view of the navicular bone.

BEAM CENTER: Over middle of heel bulbs

PROXIMAL PHALANGES

Lateral View (Short and Long Pastern)

The foot of the patient is placed on a wood block so that it is elevated slightly off the floor. The cassette is placed next to the medial aspect of the foot and should be on and perpendicular to the floor. The limb of interest should be weight bearing (Figs. 19-15 and 19-16). It may be necessary to raise the opposite limb to eliminate motion. The x-ray beam is directed horizontally toward the phalanx. The field of view should include the first and second phalanges for a general projection of the area.

Figure 19-15 Correct positioning for the lateral view of the proximal phalanges.

Figure 19-16 Radiograph of the lateral view of the proximal phalanges.

BEAM CENTER: Over area of interest

Dorsopalmar/Dorsoplantar View

The patient is positioned so that the limb under examination is weight bearing. The cassette is placed behind the limb parallel to the phalanges (Figs. 19-17 and 19-18). It may be necessary to elevate the opposite limb to minimize motion. Depending on the angle of the foot and the placement of the cassette, it may be necessary to direct the x-ray tube at a 30- to 45-degree angle to the floor. The x-ray beam must be perpendicular to the cassette. The field of view should include the first and second phalanges.